Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine while engine pumping work is maintained near the pumping work of a normally aspirated engine of similar displacement. Thus, turbo charging can extend the operating region of an engine. However, turbocharged engines can have difficulty obtaining rapid catalyst light-off times after an engine start. The additional mass and surface area introduced by the turbine housing can reduce the catalyst inlet temperature significantly. Previous solutions to achieve rapid catalyst warm-up have relied on high heat flux combustion strategies and/or turbine bypass valves to increase the temperature of exhaust reaching the catalyst.
However, the inventors herein have recognized a few issues with the above approach. High heat combustion utilizes excess fuel, reducing fuel economy. Further, the use of turbine bypass valves can be complex and may pose sealing and high actuation force requirements.
Thus, in one example, some of the above issues may be at least partly addressed by an engine method comprising, during a first condition, firing a subset of cylinders and routing all exhaust from the subset of cylinders through a first exhaust manifold coupled directly to a catalyst and not a turbocharger, and during a second condition, firing all cylinders, routing a first portion of exhaust through a second exhaust manifold coupled to the turbocharger, and routing a second portion of exhaust through the first exhaust manifold.
In this way, during cold engine start conditions, exhaust from the fired cylinders may be directly routed to the catalyst while bypassing the turbine. The engine may be configured such that the exhaust port surface area between the exhaust valves and catalyst face is reduced, for example, the engine may be an inline four cylinder engine with one exhaust port from each of the inner cylinders coupled directly to the catalyst via the first exhaust manifold. During the cold start operation, a camshaft profile may be set such that only the exhaust ports coupled to the first exhaust manifold are opened. Then, during standard, warmed up operation, all the cylinders are fired and the camshaft profile switched so that each exhaust port of each cylinder is opened, allowing a majority of the exhaust to be directed to the turbine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.